Owing to fluorescence properties and small size, graphene quantum dots (GQDs) have been investigated for biomedical imaging and drug delivery. Although GQDs demonstrate some desirable properties, poor stability and low biocompatility have limited their applications in various aspects. Therefore, in this study, we synthesized surface-passivated GQDs and studied their properties. GQDs were prepared using thermal exfoliation of graphite oxide, followed nitric acid refluxing. Then GQDs were passivated with polyethylene glycol (PEG) using hydrothermal at 120 ℃ for 72 h. Both GQDs and PEG-GQDs were characterized by transmission electron microscopy (TEM),UV-Vis absorption and photoluminescence spectroscopy. Compared with GQDs, PEG-GQDs exhibit higher fluorescence intensity and fluorescent quantum yield, which indicates that PEG-GQDs are more desirable for biomedical imaging. Also, PEG-GQDs demonstrate less cytotoxicity than GQDs in terms of cell survival rate in MTT assay. Our findings demonstrate that both fluorescence characteristics and biocompatibility of GQDs can be improved by PEG surface passivation.